The Cambridge Turning Trials

After a bit of a break, I've done another set of trials; this time on silver steel. The silver steel used was 25mm diameter, recently bought in my local professional tool warehouse. There's no direct equivalent in Machinery's Handbook, but I think that the tool steel W1 is a fairly close match; W1 is a high carbon plain steel for water hardening. For W1 the recommended speeds are 100fpm for HSS and 350fpm for uncoated carbide. For these trials I used speeds of 370rpm and 1200rpm. The insert used was a 0.2mm radius insert from Korloy, grade NC3120 for steel. In view of the small nose radius in each case depth of cut was 20 thou and a feed rate of 2 thou/rev. I tried four combinations:

Edited By Andrew Johnston on 21/05/2012 21:59:44

Love this.

Nice to see some original research.

thanks for sharing

Perhaps the banding is there all the time but the finer finish with coolant makes it more visible. I've noticed banding occasionally but it is always at leadscrew pitch. I have also noticed it can appear on an otherwise good finish when polishing after turning.

I' ve always assumed that it is caused by a slight rocking of the saddle on it's ways but stand to be corrected.

Russell.

These result concur with my long held view and experience, that if you get the cutting conditions correct with insert tooling, the need for coolant is very much rediced. Colour of the swarf (without coolant) is always a good indication of the near correct conditions.

Thank you Andrew for your ongoing commitment to these trials.

I'm puzzled by the banding. I've never seen it before over a wide range of materials and tooling, both HSS and carbide. Mind you I almost never polish after turning; may be that's why I haven't seen it before. The leadscrew pitch on my lathe is 1/4", and the banding was roughly every 40 thou, so may be not the cause? The banding was a difference in colour, metallic and dark, rather than a mechanical feature, almost like staining. I can't see what is special about silver steel that should cause it to stain. Could be a heating/cooling effect I suppose, but coolant means flood coolant, not a dribble, so why does it appear at this particular repetition rate? Anybody got any other ideas?

If I get time this week I'll have another go and see if I can reproduce it. If so, I'll try and examine it in more detail.

Regards,

Andrew

Hi Andrew, like Russell suggests, maybe it is there all time. Could it be a metallurgical phenomenon that is caused during manufacture? If you were to normalise a piece and do the same trial, would you get the same banding effect?

Just a thought.

Regards Nick.

Here is a picture of the results of another quick trial. The first 1/2", from the right, was run at 40 thou doc, 2 thou/rev, 1200rpm and no coolant. The remainder up to the first step is identical, except for coolant. The last step, on the left, was 20 thou doc, with coolant, the other parameters the same.

The finish with no coolant is torn under the magnifying glass, the other finishes are not torn, but there is a definite and regular difference in the finish, which shows up as the banding. Surface roughness measurements, from the right, are 3.15µm, 1.80µm and 2.16µm respectively. Certainly on the results from this quick test I'd use coolant on silver steel. All very confusing.

I guess the next thing to try is a different bar, from a different supplier.

Regards,

Andrew

Good question; and the answer is......neither. Close examination with a magnifying glass shows that the banding is in fact a multi-start thread. I counted eight starts. That implies that the phenomenon that is causing the banding is to some extent regular in time, and at a rate about eight times the spindle speed. Whether it is actually locked to the spindle speed is another matter.

I did another couple of tests. One at 1700rpm with coolant; the banding is still there. It's difficult to tell if it is at a different spacing. I also tried a test at 1200rpm, with coolant, on a different bar, albeit from the same source. This also exhibited some banding, but much less obvious. You have to get the light on it at the correct angle to see the banding.

On the next test I'll dig out a piece of genuine Peter Stubs silver steel and try that.

Regards,

Andrew

Eight starts? Probably a resonance or slight misalignment in the auto-feed setup.

Neil

Andrew

I have noticed a similar effect on some of the pieces I have turned. I have always put it down to the fact that as I cut using the manual feed on the top slide then it is caused by my turning of the handle which causes slight variations of pressure applied by the cutting tool, even when trying to be ultra careful. Again on rough cuts it doesn't seem to be apparent but on some fine/finishing cuts it can appear.

I wonder if in fact it is there all the time but only becomes really visible when the surface finish is smooth enough, finger marks on glass syndrome. I have seen similar when milling, both end mill and fly cutting, as any slight imperfections appear as the finish gets more mirror like.

Just a thought

Martin

Edited By Martin W on 02/06/2012 23:44:21

Hi

I have suffered from this banding phenomenon for as long as I have used my lathe and it can be frustrating when trying to get a really good finish. I have noticed it on most materials.

I have not investigated it in any depth, I assume the following may be contributing to the problem.

1. I am using a cheap Chinese lathe, so nothing is going to be perfect, poorly fitting surfaces/slides will be more effected by vibration than on a good quality machine.

2. The lathe is of light construction so has less vibration dampening effect.

3. There are various sinusoidal frequencies taking place from the motor, pulleys, gears in the feed train etc. each with there own frequency, these vibration frequencies periodically coincide and then separate.

4. Acting as such these frequencies create a pattern of vibration, the light construction and low quality slides/bearings makes the machine more susceptible to being effected which shows up in the work piece.

I have thought about re-mounting the motor separate from the lathe to reduce one source of vibration. Slightly eccentric pulleys, out of balance rotating parts, poor quality drive belts could all be contributing to sinusoidal frequencies in the machine.

So good quality comes down to hand finishing skills more than then machining quality, not a bad thing.

Nigel

PS. This spell checker is a great improvement.

Andrew,

You have mentioned how (under certain conditions), the swarf leaves the workpiece in short pieces. Along with Nigel’s (tractionengine42) thoughts, could the swarf be curling back on itself in some sort of regular pattern, and in doing so, rubbing against the workpiece?

Just a thought.

Sam

Stubs steel is very tough and I have always had trouble getting a good finish on it, I end up using a buff stick to polish down to size.Turning is always ragged and I have at times resorted to tool post grinding on the lathe for good bearing fits.It does not seem to matter what angles you grind the tool or what materiel the tool is , it still rags up.It will be interesting to see an answer to the problem.Regarding the finish I think that it is so tough that the tool has to shear its way through the metal. This raises the bad surface finish.

Clive

If you use an insert tip intended for stainless steel, the swarf curls away nicely and leaves a smooth finish, cutting speed is critical. Just finished threading 16 x 1 mm thread on silver steel, with neat oil as coolant. Good smooth thread.

I don't know what sort of motor you use but the biggest improvement I have made in surface finish from both my lathe and mill has been to replace the single phase moters with three phase and inverters. The torque of q three phase motor is (virtually) independent of the shaft angle.

Russell.

Sorted! Thanks one and all for the suggestions; they helped me clarify my thoughts and identify the problem. A special mention for Nigel, who, in essence, identified the actual issue.

A quick look at a number of previous turning exercises showed a very slight banding on stainless steel, but on nothing else as far as I could see. This tells us the problem is not specifically related to silver steel, but may be related to tougher materials, and hence higher cutting forces. My lathe is a Harrison M300, on the makers stand, and solidly bolted to a concrete floor. It is three phase and runs from a true three phase supply. The chuck is a Burnerd high speed collet chuck. Thus I discounted vibration from the machine itself, or the main motor. I calculated a few frequencies for spindle speed and the rough frequency of the banding, but no obvious resonances sprang to mind. I didn't do the calculations for the drive train, as the fine feeds are driven by a separate drive shaft rather than the leadscrew.

Now what I didn't tell you is that the cross slide is a fairly easy fit, and one of the bolts that holds the top slide down is a bit iffy. Yes, I know it's obvious in retrospect but us country boys are a bit slow on t'uptake.

The task this morning was to make and fit two new T-bolts for the top slide, like this:

As an aside I discovered that the cross slide tapered gib has a special brass insert and grub screw pressing on it from the side of the cross slide, as well as the normal adjustment screws each end. I never knew that! It's not mentiond in the manual either, although it is shown in the exploded parts drawing.

With everything cleaned, adjusted and re-assembled I tried another couple of tests on the original silver steel bar. Both tests were done at 1200rpm, 0.2mm radius insert, depth of cut 20 thou, and feedrate 2 thou/rev, one test with coolant, one without. In both cases the swarf came off as a long open spiral. Both surfaces look pretty uniform under the magnifying glass, with just a hint of tearing in places, although the finish doesn't feel rough. Surface roughness measurements (Ra, µm) are:

With coolant: 2.15 1.72 2.10 Average=1.99

Without coolant: 1.40 1.55 1.40 Average=1.45

So in this case without coolant seems better, which is what I would have originally expected.

As is so often the case the solution to what seems to be a mysterious effect is actually very simple, in this case operator stupidity. I should have fixed the iffy bolt issue long ago.

Regards,

Andrew

I think it was in the early 1990s, an artical in MEabout a similar problem, It was reduced, first bu mounting the motor on a rubber pad, with rubber bushes for the mounting bolts, isolating the motor from the lathe metalicly, then from what I remember the belts were changed for link belts. The motor was single phase, and three phase was concidered to be the best way to overcome vibration, but the isolation system was the cheapest. Ian S C

Firsly, apologies to Andrew for hijacking this thread but I thought this was the best place for this rather than start another!

Pictures below are on a piece of unknown BMS (originally 35mm  

Only 2 feeds used, 2 thou and 10 thou, depths 1/2 thou and 5 thou, speeds 600rpm and 1800 rpm, all mixed and matched but with a smear of neat cutting fluid applied first..

My deductions are that I'm more confused now than before!

Undoubtably rigidity has a lot to do with it as the inserts just scraped on the surface making a right mess, speed, depth, feed, nothing made any difference.

But chipbreaker form did!

The middle section was using a high positive rake insert, 1/2 thou depth at 600 rpm. My diamond toolholder turned the 'ring' at the righthand side end (finish is better than it looks in the picture) but high rake on a less rigid machine seems to be the answer for me at least as far as surface finish is concerned.

(I also now have a decent collection of about 15 different types of 0.4mm CCMT inserts)

Paul

Edited By KWIL on 02/07/2012 18:37:35

Paul: Feel free to add to the thread at any time. The finish on the central part looks very good. Can you expand on the high positive rake insert; manufacturer, bought from where?

KWIL: Well blow me down, so that's what it is for! Us country yokels might be slow, but we do get there in the end. Where's the lock on the top slide? I normally just tighten up the gib strip on the top slide so it can't be moved; unless I need to use it.

Regards,

Andrew